1. Introduction
This document provides a complement to the threat analysis for
Multipath TCP (MPTCP) [RFC6824] documented in RFC 6181 [RFC6181].
RFC 6181 provided a threat analysis for the general solution space of
extending TCP to operate with multiple IP addresses per connection.
Its main goal was to leverage previous experience acquired during the
design of other multi-address protocols, notably Shim6 [RFC5533], the
Stream Control Transmission Protocol (SCTP) [RFC4960], and Mobile
IPv6 (MIP6) [RFC6275] for designing MPTCP. Thus, RFC 6181 was
produced before the actual MPTCP specification (RFC 6824) was
completed and documented a set of recommendations that were
considered during the production of that specification.
This document complements RFC 6181 with a vulnerability analysis of
the mechanisms specified in RFC 6824. The motivation for this
analysis is to identify possible security issues with MPTCP as
currently specified and propose security enhancements to address
these identified security issues.
The goal of the security mechanisms defined in RFC 6824 was to make
MPTCP no worse than currently available single-path TCP. We believe
that this goal is still valid, so we will perform our analysis on the
same grounds. This document describes all the threats identified
that are specific to MPTCP (as defined in RFC 6824) that are not
possible with single-path TCP. This means that threats that are
common to TCP and MPTCP are not covered in this document.
For each attack considered in this document, we identify the type of
attacker. We can classify the attackers based on their location as
follows:
o Off-path attacker. This is an attacker that does not need to be
located in any of the paths of the MPTCP session at any point in
time during the lifetime of the MPTCP session. This means that
the off-path attacker cannot eavesdrop any of the packets of the
MPTCP session.
o Partial-time on-path attacker. This is an attacker that needs to
be in at least one of the paths during part of the lifetime of the
MPTCP session (but not the entire lifetime). The attacker can be
in the forward and/or backward directions for the initial subflow
and/or other subflows. The specific needs of the attacker will be
made explicit in the attack description.

o On-path attacker. This attacker needs to be on at least one of
the paths during the whole duration of the MPTCP session. The
attacker can be in the forward and/or backward directions for the
initial subflow and/or other subflows. The specific needs of the
attacker will be made explicit in the attack description.
We can also classify the attackers based on their actions as follows:
o Eavesdropper. The attacker is able to capture some of the packets
of the MPTCP session to perform the attack, but it is not capable
of changing, discarding, or delaying any packet of the MPTCP
session. The attacker can be in the forward and/or backward
directions for the initial subflow and/or other subflows. The
specific needs of the attacker will be made explicit in the attack
description.
o Active attacker. The attacker is able to change, discard, or
delay some of the packets of the MPTCP session. The attacker can
be in the forward and/or backward directions for the initial
subflow and/or other subflows. The specific needs of the attacker
will be made explicit in the attack description.
In this document, we consider the following possible combinations of
attackers:
o an on-path eavesdropper
o an on-path active attacker
o an off-path active attacker
o a partial-time on-path eavesdropper
o a partial-time on-path active attacker
In the rest of the document, we describe different attacks that are
possible against the MPTCP protocol specified in RFC 6824 and propose
possible security enhancements to address them.
2. ADD_ADDR Attack
Summary of the attack:
Type of attack: MPTCP session hijack enabling a man-in-the-middle
(MitM) attack
Type of attacker: off-path active attacker

Description:
In this attack, the attacker uses the ADD_ADDR option defined in RFC
6824 to hijack an ongoing MPTCP session and enable himself to perform
a man-in-the-middle attack on the MPTCP session.
Consider the following scenario. Host A with address IPA has one
MPTCP session with Host B with address IPB. The MPTCP subflow
between IPA and IPB is using port PA on Host A and port PB on Host B.
The tokens for the MPTCP session are TA and TB for Host A and Host B,
respectively. Host C is the attacker. It owns address IPC. The
attack is executed as follows:
1. Host C sends a forged packet with source address IPA, destination
address IPB, source port PA, and destination port PB. The packet
has the ACK flag set. The TCP sequence number for the segment is
i, and the ACK sequence number is j. We will assume all these
are valid; later, we discuss what the attacker needs to figure
them out. The packet contains the ADD_ADDR option. The ADD_ADDR
option announces IPC as an alternative address for the
connection. It also contains an 8-bit address identifier that
does not provide any strong security benefit.
2. Host B receives the ADD_ADDR message and replies by sending a TCP
SYN packet.
Note: The MPTCP specification [RFC6824] states that the host
receiving the ADD_ADDR option may initiate a new subflow. If
the host is configured so that it does not initiate a new
subflow, the attack will not succeed. For example, on the
current Linux implementation, the server does not create
subflows. Only the client does so.
The source address for the packet is IPB; the destination address
for the packet is IPC; the source port is PB'; and the
destination port is PA' (it is not required that PA=PA' nor that
PB=PB'). The sequence number for this packet is the new initial
sequence number for this subflow. The ACK sequence number is not
relevant as the ACK flag is not set. The packet carries an
MP_JOIN option and the token TA. It also carries a random nonce
generated by Host B called RB.
3. Host C receives the SYN+MP_JOIN packet from Host B and alters it
in the following way. It changes the source address to IPC and
the destination address to IPA. It sends the modified packet to
Host A, impersonating Host B.

4. Host A receives the SYN+MP_JOIN message and replies with a
SYN/ACK+MP_JOIN message. The packet has source address IPA and
destination address IPC, as well as all the other needed
parameters. In particular, Host A computes a valid Hashed
Message Authentication Code (HMAC) and places it in the MP_JOIN
option.
5. Host C receives the SYN/ACK+MP_JOIN message and changes the
source address to IPC and the destination address to IPB. It
sends the modified packet to IPB, impersonating Host A.
6. Host B receives the SYN/ACK+MP_JOIN message. Host B verifies the
HMAC of the MP_JOIN option and confirms its validity. It replies
with an ACK+MP_JOIN packet. The packet has source address IPB
and destination address IPC, as well as all the other needed
parameters. The returned MP_JOIN option contains a valid HMAC
computed by Host B.
7. Host C receives the ACK+MP_JOIN message from B and alters it in
the following way. It changes the source address to IPC and the
destination address to IPA. It sends the modified packet to Host
A, impersonating Host B.
8. Host A receives the ACK+MP_JOIN message and creates the new
subflow. At this point, the attacker has managed to place itself
as a MitM for one subflow for the existing MPTCP session. It
should be noted that the subflow between addresses IPA and IPB
that does not flow through the attacker still exists, so the
attacker has not completely intercepted all the packets in the
communication (yet). If the attacker wishes to completely
intercept the MPTCP session, it can do the following additional
step.
9. Host C sends two TCP RST messages. One TCP RST packet is sent to
Host B, with source address IPA, destination address IPB, and
source and destination ports PA and PB, respectively. The other
TCP RST message is sent to Host A, with source address IPB,
destination address IPA, and source and destination ports PB and
PA, respectively. Both RST messages must contain a valid
sequence number. Note that figuring the sequence numbers to be
used here for subflow A is the same difficulty as being able to
send the initial ADD_ADDR option with valid sequence number and
ACK value. If there are more subflows, then the attacker needs
to find the sequence number and ACK for each subflow. At this
point, the attacker has managed to fully hijack the MPTCP
session.

Information required by the attacker to perform the described attack:
In order to perform this attack the attacker needs to guess or know
the following pieces of information. The attacker needs this
information for one of the subflows belonging to the MPTCP session.
o the four-tuple {Client-side IP Address, Client-side Port, Server-
side Address, Server-side Port} that identifies the target TCP
connection
o a valid sequence number for the subflow
o a valid ACK sequence number for the subflow
o a valid address identifier for IPC
TCP connections are uniquely identified by the four-tuple {Source
Address, Source Port, Destination Address, Destination Port}. Thus,
in order to attack a TCP connection, an attacker needs to know or be
able to guess each of the values in that four-tuple. Assuming the
two peers of the target TCP connection are known, the Source Address
and the Destination Address can be assumed to be known.
Note: In order to be able to successfully perform this attack, the
attacker needs to be able to send packets with a forged source
address. This means that the attacker cannot be located in a
network where techniques like ingress filtering [RFC2827] or
source address validation [RFC7039] are deployed. However,
ingress filtering is not as widely implemented as one would expect
and hence cannot be relied upon as a mitigation for this kind of
attack.
Assuming the attacker knows the application protocol for which the
TCP connection is being employed, the server-side port can also be
assumed to be known. Finally, the client-side port will generally
not be known and will need to be guessed by the attacker. The
chances of an attacker guessing the client-side port will depend on
the ephemeral port range employed by the client and whether or not
the client implements port randomization [RFC6056].
Assuming TCP sequence number randomization is in place (see e.g.,
[RFC6528]), an attacker would have to blindly guess a valid TCP
sequence number. That is,
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT =<
SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

As a result, the chances of an attacker succeeding will depend on the
TCP receive window size at the target TCP peer.
Note: Automatic TCP buffer tuning mechanisms have become common
for popular TCP implementations; hence, very large TCP window
sizes of values up to 2 MB could end up being employed by such TCP
implementations.
According to [RFC793], the acknowledgement number is considered valid
as long as it does not acknowledge the receipt of data that has not
yet been sent. That is, the following expression must be true:
SEG.ACK <= SND.NXT
However, for implementations that support [RFC5961], the following
(stricter) validation check is enforced:
SND.UNA - MAX.SND.WND <= SEG.ACK <= SND.NXT
Finally, in order for the address identifier to be valid, the only
requirement is that it needs to be different from the ones already
being used by Host A in that MPTCP session, so a random identifier is
likely to work.
Given that a large number of factors affect the chances of an
attacker successfully performing the aforementioned off-path attacks,
we provide two general expressions for the expected number of packets
the attacker needs to send to succeed in the attack: one for MPTCP
implementations that support [RFC5961] and another for MPTCP
implementations that do not.
Implementations that do not support RFC 5961:
Packets = (2^32/(RCV_WND)) * 2 * EPH_PORT_SIZE/2 * 1/MSS
Where the new parameters are:
Packets:
Maximum number of packets required to successfully perform an off-
path (blind) attack.
RCV_WND:
TCP receive window size (RCV.WND) at the target node.

SND_MAX_WND:
Maximum TCP send window size ever employed by the target MPTCP
endpoint (MAX.SND.WND).
EPH_PORT_SIZE:
Number of ports comprising the ephemeral port range at the
"client" system.
Notes:
The value "2^32" represents the size of the TCP sequence number
space.
The parameter "MAX.SND.WND" is specified in [RFC5961].
The value "2 * SND_MAX_WND" results from the expression "SND.NXT -
SND.UNA - MAX.SND.WND", assuming that, for connections that
perform bulk data transfers, "SND.NXT - SND.UNA == MAX.SND.WND".
If an attacker targets a TCP endpoint that is not actively
transferring data, "2 * SND_MAX_WND" would become "SND_MAX_WND"
(and hence a successful attack would typically require more
packets).
The following table contains some sample results for the number of
required packets, based on different values of RCV_WND, SND_MAX_WND,
and EPH_PORT_SIZE. For these implementations, only a limited number
of sample results are provided (as an indication of how [RFC5961]
increases the difficulty of performing these attacks).
+-------------+-------------+-----------+-----------+---------+
| Ports \ Win | 16 KB | 128 KB | 256 KB | 2048 KB |
+-------------+-------------+-----------+-----------+---------+
| 4000 | 45812984490 | 715827882 | 178956970 | 2796202 |
+-------------+-------------+-----------+-----------+---------+
Table 2: Maximum Number of Packets for Successful Attack
Note:
In the aforementioned table, all values are computed with RCV_WND
equal to SND_MAX_WND.
2.1. Possible Security Enhancements to Prevent This Attack
1. To include the token of the connection in the ADD_ADDR option.
This would make it harder for the attacker to launch the attack,
since the attacker needs to either eavesdrop the token (so this
can no longer be a blind attack) or to guess it, but a random
32-bit number is not easy to guess. However, this would imply
that any eavesdropper that is able to see the token would be able

to launch this attack. This solution then increases the
vulnerability window against eavesdroppers from the initial 3-way
handshake for the MPTCP session to any exchange of the ADD_ADDR
messages.
2. To include the HMAC of the address contained in the ADD_ADDR
option. The key used for the HMAC is the concatenation of the
key of the receiver and the key of the sender (in the same way
they are used for generating the HMAC of the MP_JOIN message).
This makes it much more secure, since it requires the attacker to
have both keys (either by eavesdropping it in the first exchange
or by guessing it). Because this solution relies on the key used
in the MPTCP session, the protection of this solution would
increase if new key generation methods are defined for MPTCP
(e.g., using Secure Socket Layer (SSL) keys as has been
proposed).
3. To include the destination address of the SYN packet in the HMAC
of the MP_JOIN message. As the attacker requires changing the
destination address to perform the described attack, protecting
it would prevent the attack. It wouldn't allow hosts behind NATs
to be reached by an address in the ADD_ADDR option, even with
static NAT bindings (like a web server at home).
Of the options described above, option 2 is recommended as it
achieves a higher security level while preserving the required
functionality (i.e., NAT compatibility).
3. DoS Attack on MP_JOIN
Summary of the attack:
Type of attack: MPTCP denial-of-service attack, preventing the
hosts from creating new subflows
Type of attacker: off-path active attacker
Description:
As currently specified, the initial SYN+MP_JOIN message of the 3-way
handshake for additional subflows creates state in the host receiving
the message. This is because the SYN+MP_JOIN contains the 32-bit
token that allows the receiver to identify the MPTCP session and the
32-bit random nonce used in the HMAC calculation. As this
information is not re-sent in the third ACK of the 3-way handshake, a
host must create state upon reception of a SYN+MP_JOIN.

Assume that an MPTCP session exists between Host A and Host B, with
tokens TA and TB. An attacker, sending a SYN+MP_JOIN to Host B, with
the valid token TB, will trigger the creation of state on Host B.
The number of these half-open connections a host can store per MPTCP
session is limited by a certain number and is implementation-
dependent. The attacker can simply exhaust this limit by sending
multiple SYN+MP_JOINs with different 5-tuples. The (possibly forged)
source address of the attack packets will typically correspond to an
address that is not in use, or else, the SYN/ACK sent by Host B would
elicit a RST from the impersonated node, thus removing the
corresponding state at Host B. Further discussion of traditional SYN
flooding attacks and common mitigations can be found in [RFC4987].
This effectively prevents Host A from sending any more SYN+MP_JOINs
to Host B, as the number of acceptable half-open connections per
MPTCP session on Host B has been exhausted.
The attacker needs to know the token TB in order to perform the
described attack. This can be achieved if it is a partial-time on-
path eavesdropper observing the 3-way handshake of the establishment
of an additional subflow between Host A and Host B. If the attacker
is never on-path, it has to guess the 32-bit token.
3.1. Possible Security Enhancements to Prevent This Attack
The third packet of the 3-way handshake could be extended to also
contain the 32-bit token and the random nonce that has been sent in
the SYN+MP_JOIN. Further, Host B will have to generate its own
random nonce in a reproducible fashion (e.g., a hash of the 5-tuple +
initial sequence number + local secret). This will allow Host B to
reply to a SYN+MP_JOIN without having to create state. Upon the
reception of the third ACK, Host B can then verify the correctness of
the HMAC and create the state.
4. SYN Flooding Amplification
Summary of the attack:
Type of attack: The attacker uses SYN+MP_JOIN messages to amplify
the SYN flooding attack.
Type of attacker: off-path active attacker

Description:
SYN flooding attacks [RFC4987] use SYN messages to exhaust the
server's resources and prevent new TCP connections. A common
mitigation is the use of SYN cookies [RFC4987] that allow stateless
processing of the initial SYN message.
With MPTCP, the initial SYN can be processed in a stateless fashion
using the aforementioned SYN cookies. However, as described in the
previous section, as currently specified, SYN+MP_JOIN messages are
not processed in a stateless manner. This opens a new attack vector.
The attacker can now open an MPTCP session by sending a regular SYN
and creating the associated state but then sending as many
SYN+MP_JOIN messages as supported by the server with different
combinations of source address and source port, consuming the
server's resources without having to create state in the attacker.
This is an amplification attack, where the cost on the attacker side
is only the cost of the state associated with the initial SYN while
the cost on the server side is the state for the initial SYN plus all
the state associated with all the following SYN+MP_JOINs.
4.1. Possible Security Enhancements to Prevent This Attack
1. The solution described for the previous DoS attack on MP_JOIN
would also prevent this attack.
2. Limiting the number of half-open subflows to a low number (e.g.,
three subflows) would also limit the impact of this attack.
5. Eavesdropper in the Initial Handshake
Summary of the attack:
Type of attack: An eavesdropper present in the initial handshake
where the keys are exchanged can hijack the MPTCP session at any
time in the future.
Type of attacker: partial-time on-path eavesdropper
Description:
In this case, the attacker is present along the path when the initial
3-way handshake takes place and therefore is able to learn the keys
used in the MPTCP session. This allows the attacker to move away
from the MPTCP session path and still be able to hijack the MPTCP
session in the future. This vulnerability was readily identified
when designing the MPTCP security solution [RFC6181], and the threat
was considered acceptable.

5.1. Possible Security Enhancements to Prevent This Attack
There are many techniques that can be used to prevent this attack,
and each of them represents different trade-offs. At this point, we
limit ourselves to enumerate them and provide useful pointers.
1. Use of hash chains. The use of hash chains for MPTCP has been
explored in [HASH-CHAINS].
2. Use of SSL keys for MPTCP security as described in [MPTCP-SSL].
3. Use of Cryptographically Generated Addresses (CGAs) for MPTCP
security. CGAs [RFC3972] have been used in the past to secure
multi-addressed protocols like Shim6 [RFC5533].
4. Use of tcpcrypt [TCPCRYPT].
5. Use of DNSSEC. DNSSEC has been proposed to secure the Mobile IP
protocol [DNSSEC].
6. SYN/JOIN Attack
Summary of the attack:
Type of attack: An attacker that can intercept the SYN/JOIN
message can alter the source address being added.
Type of attacker: partial-time on-path eavesdropper
Description:
The attacker is present along the path when the SYN/JOIN exchange
takes place. This allows the attacker to add any new address it
wants to by simply substituting the source address of the SYN/JOIN
packet for one it chooses. This vulnerability was readily identified
when designing the MPTCP security solution [RFC6181], and the threat
was considered acceptable.
6.1. Possible Security Enhancements to Prevent This Attack
It should be noted that this vulnerability is fundamental due to the
NAT support requirement. In other words, MPTCP must work through
NATs in order to be deployable in the current Internet. NAT behavior
is unfortunately indistinguishable from this attack. It is
impossible to secure the source address, since doing so would prevent
MPTCP from working through NATs. This basically implies that the
solution cannot rely on securing the address. A more promising
approach would be to look into securing the payload exchanged and

thus limiting the impact that the attack would have in the
communication (e.g., tcpcrypt [TCPCRYPT] or similar).
7. Recommendations
The current MPTCP specification [RFC6824] is Experimental. There is
an ongoing effort to move it to Standards Track. We believe that the
work on MPTCP security should follow two threads:
o The work on improving MPTCP security so that the MPTCP
specification [RFC6824] can become a Standards Track document.
o The work on analyzing possible additional security enhancements to
provide a more secure version of MPTCP.
We expand on these in the following subsections.
7.1. MPTCP Security Improvements for a Standards Track Specification
We believe that in order for MPTCP to progress to Standards Track,
the ADD_ADDR attack must be addressed. We believe that the solution
that should be adopted in order to deal with this attack is to
include an HMAC to the ADD_ADDR message (with the address being added
used as input to the HMAC as well as the key). This would make the
ADD_ADDR message as secure as the JOIN message. In addition, this
implies that if we implement a more secure way to create the key used
in the MPTCP connection, then the security of both the MP_JOIN and
the ADD_ADDR messages is automatically improved (since both use the
same key in the HMAC).
We believe that this is enough for MPTCP to progress as a Standards
Track document because the security level is similar to single-path
TCP per our previous analysis. Moreover, the security level achieved
with these changes is exactly the same as other Standards Track
documents. In particular, this would be the same security level as
SCTP with dynamic addresses as defined in [RFC5061]. The Security
Considerations section of RFC 5061 (which is a Standards Track
document) reads:
The addition and or deletion of an IP address to an existing
association does provide an additional mechanism by which existing
associations can be hijacked. Therefore, this document requires
the use of the authentication mechanism defined in [RFC4895] to
limit the ability of an attacker to hijack an association.
Hijacking an association by using the addition and deletion of an
IP address is only possible for an attacker who is able to
intercept the initial two packets of the association setup when

the SCTP-AUTH extension is used without pre-shared keys. If such
a threat is considered a possibility, then the [RFC4895] extension
MUST be used with a preconfigured shared endpoint pair key to
mitigate this threat.
This is the same security level that would be achieved by MPTCP with
the addition of the ADD_ADDR security measure recommended in this
document.
7.2. Security Enhancements for MPTCP
We also believe that is worthwhile to explore alternatives to secure
MPTCP. As we identified earlier, the problem of securing JOIN
messages is fundamentally incompatible with NAT support, so it is
likely that a solution to this problem involves the protection of the
data itself. Exploring the integration of MPTCP and approaches like
tcpcrypt [TCPCRYPT] and exploring integration with SSL seem
promising.
8. Security Considerations
This whole document is about security considerations for MPTCP.
9. References
9.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<http://www.rfc-editor.org/info/rfc3972>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <http://www.rfc-editor.org/info/rfc4895>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<http://www.rfc-editor.org/info/rfc5061>.